There is developed, as a fuel economy improvement of a vehicle, an art to perform a fuel cut when an engine speed is equal to or greater than a predetermined value in a coast traveling state in which an accelerator pedal is released. Moreover, a torque converter is locked-up during the fuel cut. When a vehicle speed is decreased during this fuel cut, the engine speed is decreased. When the engine speed is decreased to be a fuel recovery rotation speed, a fuel supply recovery (fuel recovery) is performed. At this fuel recovery, the lockup of the torque converter is previously released.
However, it is preferable to extend (prolong) the fuel cut period as long as possible, for improving the fuel economy.
Accordingly, there is developed an art to extend (prolong) the fuel cut period by downshifting the automatic transmission in accordance with the decrease of the vehicle speed to suppress the decrease of the engine speed to the fuel recovery rotation speed, as described in a patent document 1 (paragraph 0036 and so on) and so on.
Incidentally, a shift stage of the automatic transmission is preferred to be downshifted one after another in accordance with the decrease of the vehicle speed, for extending (prolonging) the fuel cut period as long as possible.
For example, FIG. 9 is a time chart showing one example of a downshift control for extending (prolonging) the fuel cut period as long as possible. In the drawing, NxtGp represents the shift stage. Ne is the engine speed. NLUre is a lockup release rotation speed. When the engine speed Ne is decreased to be this rotation speed NLUre, the lockup of the torque converter is released. NFCR is the fuel recovery rotation speed. When the engine speed Ne is decreased to be this rotation speed FCR after the release of the lockup, the fuel recovery is performed.
As shown in FIG. 9, when the engine speed Ne is decreased to approach the lockup release rotation speed NLUre due to the decrease of the vehicle speed (time t1), the downshift is performed. The engine speed Ne is decreased during a time period (during a torque phase) from the command of the downshift at time t1 to time t2 at which an inertia phase of the downshift is started. Accordingly, the downshift is commanded with a margin for this decrease of the engine speed Ne.
The engine speed Ne is increased until time t3 at which the downshift is finished by the increase of the transmission gear ratio by the inertia phase of the downshift. However, the engine speed Ne is decreased thereafter in accordance with the decrease of the vehicle speed. When the engine speed Ne is decreased to approach the lockup release rotation speed NLUre again (time t4), the downshift is performed.
In this way, by the sequential downshift according to the decrease of the vehicle speed, it is possible to extend (prolong) the fuel cut period while maintaining the lockup, and to improve the fuel economy.
However, for example, in a case where a strong brake force is added during the coast traveling of the vehicle, the downshift operation cannot follow the decrease of the vehicle speed.
That is, in case of the large deceleration of the vehicle, a decrease amount of the engine speed according to the decrease of the vehicle speed during the time period during which the downshift is performed becomes greater than the increase amount by the recovery of the engine speed by the downshift. Accordingly, the engine speed is decreased to be equal to or smaller than the lockup release rotation speed NLUre during the next downshift, so that the lockup and the fuel cut cannot be continued.